As will be apparent from this publication, plastifying apparatus of this type may also be known as a double-worm extruder, a double-screw extruder, a compounding apparatus or unit and by terms of similar import. In the following discussion, I may refer to such a unit simply as an extruder.
An apparatus of this type is equipped with two worms or screws which can be driven in the same sense or in opposite senses and which can have interfitting flights or ribs, i.e. the screws can be provided adjacent one another so that they effectively interfit or mesh.
Along the path of the material to be thermoplastified along the screws and in the chambers of the housing receiving the screws, throttle locations can be provided at which a steplessly variable change in the cross section can be provided.
This change in the cross section varies the flow velocity of the material through the apparatus and, to optimize the plastification, can be modified to suit the particular material. The adjustment of the throttle can also be simply matched to the rheological conditions to the material which is processed, especially the flow characteristics and viscosities thereof. Adjustment at the throttles can allow the residence time of the material in the apparatus and the degree of filling of the material in the apparatus to be varied and thereby enable adjustment of the degree of plastification of the material.
The publication cited describes two extruders with different constructions of the throttle. One version provides a radial throttle in which slide plates can be introduced in a radial direction into the screw housing to a greater or lesser depth. Such slide plates produce dead zones in the flow of the melt which remain in the product downstream of the plates, can form residues which constitute contaminants thereof and can lead to defects in the product.
In an axial throttle version, a radial narrowing of the throttle gap is effected by axial shifting of the screw. This also varies the length of the throttle region. Because of the relatively complex shape of the screw housing and the screw and the need for axial shifting of the screw, the apparatus is inordinately expensive and more difficult to maintain.